Django - annotate with multiple Count - django

Question

I have a model called Post which has two fields upvotes and downvotes. Now, upvotes, downvotes are ManyToManyField to a Profile. This is the model:
class Post(models.Model):
profile = models.ForeignKey(Profile, on_delete=models.CASCADE)
title = models.CharField(max_length=300)
content = models.CharField(max_length=1000)
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now=True)
subreddit = models.ForeignKey(Subreddit, on_delete=models.CASCADE)
upvotes = models.ManyToManyField(Profile, blank=True, related_name='upvoted_posts')
downvotes = models.ManyToManyField(Profile, blank=True, related_name='downvoted_posts')
So, I want to fetch all the posts such that they are in the order of
total(upvotes) - total(downvotes)
So I have used this query:
Post.objects.annotate(
total_votes=Count('upvotes')-Count('downvotes')
).order_by('total_votes')
The problem with this query is the total_votes is always turning out to be zero.
The below queries will explain the situation:
In [5]: Post.objects.annotate(up=Count('upvotes')).values('up')
Out[5]: <QuerySet [{'up': 1}, {'up': 3}, {'up': 2}]>
In [6]: Post.objects.annotate(down=Count('downvotes')).values('down')
Out[6]: <QuerySet [{'down': 1}, {'down': 1}, {'down': 1}]>
In [10]: Post.objects.annotate(up=Count('upvotes'), down=Count('downvotes'), total=Count('upvotes')-Count('downvotes')).values('up', 'down', 'total')
Out[10]: <QuerySet [{'up': 1, 'down': 1, 'total': 0}, {'up': 3, 'down': 3, 'total': 0}, {'up': 2, 'down': 2, 'total': 0}]>
Seems like both up and down are having the same value(which is actually the value of up). How can I solve this?
I have tried this:
In [9]: Post.objects.annotate(up=Count('upvotes')).annotate(down=Count('downvotes')).values('up', 'down')
Out[9]: <QuerySet [{'up': 1, 'down': 1}, {'up': 3, 'down': 3}, {'up': 2, 'down': 2}]>
but even this gives the same output.

Answer 1

Try to use dictinct argument:
Post.objects.annotate(
total_votes=Count('upvotes', distinct=True)-Count('downvotes', distinct=True)
).order_by('total_votes')
From the docs:
Combining multiple aggregations with annotate() will yield the wrong
results because joins are used instead of subqueries. For most
aggregates, there is no way to avoid this problem, however, the Count
aggregate has a distinct parameter that may help.

Answer 2

(I'm aware that this isn't exactly an answer, but code can't be embedded in a comment.)
A better data model would be
class Post:
# ...
class Vote:
voter = models.ForeignKey(Profile, on_delete=models.PROTECT)
post = models.ForeignKey(Post, on_delete=models.CASCADE)
score = models.IntegerField() # either -1 or +1; validate accordingly
class Meta:
unique_together = [('voter', 'post'),]
This way you could count the current total score for a post simply with
Vote.objects.filter(post=post).aggregate(score=Sum('score'))
However, you should be well aware of the performance implications of doing this (or your original version for that matter) every time. It would be better to add a
score = models.IntegerField(editable=False)
field to the Post, that gets updated with the aggregate score every time a vote is created, modified or deleted.